The mileage ratio
Miles per US gallon is distance travelled divided by US gallons burned. It is a rate, so a higher value means the car travelled farther on each gallon. The calculation assumes both numbers cover the same interval between refuelling events.
A fill-to-fill example
Driving 300 miles and then adding 10 US gallons gives 300 ÷ 10 = 30 MPG. The result describes that interval only. If the first tank was not filled to the same level as the second, the gallons are not a clean measure of what the car used.
Reset the trip meter at a full fill
For a usable log, fill the tank, reset a trip counter, drive normally, then fill again and divide recorded miles by the gallons on the receipt. A single short fill can be noisy because pump shut-off varies; averaging several full tanks smooths that noise.
Do not mix gallon systems
This page uses the US gallon. An Imperial gallon is larger, so an MPG figure reported in the UK is not directly comparable with US MPG. Convert the units before comparing a manual, a road test, or a fleet record.
When MPG conceals the change
A move from 30 to 20 MPG costs more fuel than a move from 50 to 40 MPG over the same distance, although both are a ten-MPG drop. For expense planning, litres per distance or actual gallons used can make the fuel penalty easier to see.
Boundary of the result
Mileage does not price a journey and it does not predict the next tank. Fuel grade, temperature, road speed, cargo, and driving pattern can shift the next result. Use a trip cost estimate after choosing a realistic consumption rate and fuel price.
Spotting a bad entry
A sudden 12 MPG result after several tanks near 30 does not automatically mean a mechanical fault. Check whether miles were reset, gallons were entered in litres, or one fill was partial. Recalculate from the receipt and odometer before treating an outlier as a trend.
Compare like seasons
Winter fuel formulation, cold warm-up time, and snow tyres can make a seasonal average lower than a summer average without any change in route. Labeling the date and weather context in a mileage log keeps a normal seasonal shift from being mistaken for a repair diagnosis.
A weighted tank average keeps fuel in the denominator
Two tank readings should be combined as total miles divided by total gallons, not by averaging their displayed MPG values. If one fill covers 200 miles on 10 gallons and the next covers 400 miles on 10 gallons, the readings are 20 and 40 MPG. Their arithmetic midpoint is 30 MPG, but the combined record is 600 divided by 20, also 30 only because the gallons happen to match. With 200 miles on 10 gallons and 400 miles on 20 gallons, the correct combined figure is 600 divided by 30, or 20 MPG; averaging 20 and 20 happens to work, but the total-distance method works in every case.
Read a consumption change before assigning a cause
A change from 30 to 27 MPG means fuel per 100 miles rose from 3.33 to 3.70 gallons, about 0.37 gallon more. First compare the distance source, fill level, tyre pressure condition, temperature, route, payload, and fuel units. A short interval after a partial fill is a weak observation. Several comparable full-tank entries moving in the same direction are a reason to inspect the vehicle or discuss it with a qualified technician; a calculator cannot identify a mechanical cause from a ratio alone.
Keep an estimated range separate from fuel economy
MPG can support a rough range calculation only after usable tank fuel and reserve policy are defined. At 30 MPG, a planned 12-gallon usable amount implies 360 miles under similar conditions; that is not permission to run a tank to empty. Gauge accuracy, gradients, wind, idling, detours, and a deliberate reserve change the operational answer. Record MPG as the observed rate and make any remaining-fuel assumption explicit instead of presenting one dashboard number as both economy and guaranteed distance.
Build a useful mileage history
Use a consistent fill procedure and keep each entry as date, odometer, miles since the prior comparable fill, gallons, and computed MPG. Mark unusual conditions such as towing, deep cold, long idling, a different fuel blend, or a new route. A short note lets a later reader distinguish an operating change from a measurement change. Do not overwrite an inconvenient tank result to make an average look smoother; flag a partial fill and omit it from the full-tank interval instead. The history can reveal a repeatable trend, but maintenance and safety decisions still require inspection, warnings, and manufacturer guidance beyond this arithmetic.
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It is informational only and is not medical or professional advice.